In recent years labeling techniques with fluorescent compounds have replaced the radioisotope based ones as analytical tools in biomedical applications.
Fluorescent markers allow the highly sensitive and precise measurement of components of complex biological systems and are nowadays widely used in the biological, pharmacological and medical field.
Fluorescence is the most suitable analytical tool for monitoring antigen-antibody interactions in many bioanalytical, immunochemical and histological applications, for labeling nucleic acids etc.
Several families of differently colored fluorescent markers are currently available on the market. Some of these are high molecular weight compounds, others are low molecular weight compounds with a greater capacity of penetrating the cellular membrane, others, lastly, are complex systems in which the light is emitted through a complex set of intermolecular interactions. (Handbook of Fluorescent Probes and Research Chemicals, Seventh Edition; Molecular Probes Inc., Eugene, Oreg., 1999; Fluorescent and Luminescent Probes for Biological Activity. A Practical Guide to Technology for Quantitative Real-Time Analysis, Second Edition, W. T. Mason, ed. Academic Press, 1999).
Each different family of markers requires a different chemical methodology for the bond formation between the marker and biological molecule.
Therefore, there is a need for a family of fluorescent markers in all colors of visible and possessing the same chemical characteristics, so as to be able to apply the same standard methodology for labeling in all the colors. One such family of markers would considerably help the experiments in the biomedical field, and particularly the experiments for the simultaneous monitoring of different biochemical reactions and species.
Additionally, there is a need for a class of fluorescent markers with high absorbance values, wide differences between absorption and emission frequencies and high quantum yields of photoluminescence (the ratio between the number of photons emitted and those absorbed).
Thiophene oligomers have been extensively studied in recent years due to their optic properties. In particular, thiophene oligomers have high absorbance values and broad differences between absorption and emission frequencies (H. S. Nalwa, Ed. Handbook of organic conductive molecules and polymers, John Wiley & Sons. Chichester, 1997, Vol. 1-4; Electronic Materials: The Oligomer Approach. K. Müllen, G. Wegner, Eds. Wiley-VCH, New York, 1998; Handbook of oligo and polythiophenes, D. Fichou, Ed. Wiley-VCH. New York, 1999).
Besides, it has been shown that thiophene oligomers can reach very high quantum yields of photoluminescence, both in solution as well as the solid state and that, through molecular engineering, their fluorescence frequency can be modulated in the entire spectrum of visible and the nearby IR (Oligothiophene-S, S-dioxides. Synthesis and electronic properties in relation to the parent oligothiophenes, G. Barbarella, et al. J. Org. Chem. 1998, 63, 5497-5506; High-efficiency oligothiophene-based light-emitting diodes G. Gigli, et al. Appl. Phys. Lett. 1999, 75, 439-441; Color engineering by modified oligothiophene blends, M. Anni, et al. Appl. Phys. Lett. 2000, 77, 2458-2460; Molecular packing and photoluminescence efficiency in odd-membered oligothiophene-S, S-dioxides, L. Antolini, et al. J. Am. Chem. Soc. 2000, 122, 9006-9013; Tuning solid-state photoluminescence frequencies and efficiencies of oligomers containing one central thiophene-S, S-dioxide unit G. Barbarella, et al. J. Am. Chem. Soc. 2000, 122, 11971-11978; Multicolor oligothiophene-based LEDs G. Gigli, et al. Appl. Phys. Lett. 2001, 78, 1493-1495).
Finally, it has been shown that, with suitable functionalizations, these oligomers can be made soluble not only in organic solvents but also in water (Polyhydroxyl Oligothiophenes. I. Regioselective Synthesis of 3,4′- and 3,3′-di (2-hydroxyethyl) 2,2′-bithiophene via Palladium Catalyzed Coupling of Thienylstannanes with Thienylbromides, G. Barbarella, M. Zambianchi. Tetrahedron 1994, 50, 11249-11256).